This invention relates to a printing plate and to methods of manufacturing and using a printing plate. More particularly, it relates to a lithographic printing plate which is ready for use on a printing press with minimal or no additional processing after exposure.
The production of lithographic printing plates, particularly those based upon an aluminum sheet-like support, has been well known in the graphic arts. Such printing plates are typically of the planographic type and printing is accomplished from a substantially flat surface where printing areas are neither raised appreciably above nor depressed below adjacent and surrounding non-printing areas. In general, these plates comprise hydrophobic (water-repelling) ink-receptive image areas and hydrophilic (ink-repelling) water-receptive non-image areas. The hydrophilic non-image areas are typically hydrophilic surfaces bared by an imagewise development process. Thus, non-image areas of photoresist material can, for example, be washed or otherwise removed, to bare a hydrophilic resinous layer, an aluminum (or other metal plate) surface, an anodized aluminum (or other metal plate) surface or a metal plate having a phosphate- or silicate-treated hydrophilic surface. As is described in U.S. Pat. No. 3,836,366 (issued Sep. 17, 1974 to J. L. Silver), non-image hydrophilic areas of a printing plate may undergo a loss of water receptivity because of corrosion and pitting, with the consequence that a protective hydrophilic coating, such as of gum arabic, may be applied periodically ("gumming" of the plate) to keep a protective hydrophilic coating in the non-image areas. Among disadvantages mentioned in the patent, with respect to hydrophilic resinous coatings that may be used to provide the desired hydrophilic surface, is the tendency for the hydrophilic or water-attracting character of the non-image areas to disappear gradually over extended usage and the tendency of such material to "scum", i.e., to become gradually oleophilic and ink receptive.
The gradual loss of hydrophilicity resulting from "scumming" shortens the useful life of the plate. Moreover, deterioration of the hydrophilic resinous material may not be confined to bared areas only and may extend to adjacent portions of the layer which underlie desired hydrophobic areas, with resulting "undercutting" of desired oleophilic printing areas and loss of print quality. Attempts to overcome the loss in hydrophilicity, for example, by periodic "gumming" of the plate, may in turn promote "blinding" of the plate, i.e., a reduction in hydrophobicity of ink-receptive (image) areas of the plate.
In the production of a printing plate, a development step will normally be conducted after a photo-exposure step, to remove imagewise, non-exposed or exposed regions, depending, for example, whether a negative-working or positive-working photoresist, respectively, is used over the hydrophilic surface. The development process will usually involve washing and rinsing operations which may be assisted by rubbing or brushing. Other operations such as plate "gumming" may also be performed. These operations conducted by the platemaker are conventional in the printing field and it will be appreciated, therefore, that there will be considerable attractiveness for a plate-making process that is not dependent upon the conduct of a wet development process and which permits the use of the plate directly after exposure and without further required processing.